Plain English Meaning
The lowest altitude a controller is allowed to send you to when giving you headings on radar. It is set so you stay safely above terrain and obstacles in that area, and so radar and radios still work properly.
Full Definition
The lowest altitude, expressed in feet above mean sea level (MSL), at which an air traffic controller may issue radar vectors to an aircraft, established to provide required obstacle clearance, adequate radar and communications coverage, and (in some cases) terrain separation within a defined area of controlled airspace.